You've answered your own question, Northwestern isn't a troll because they had no intention of holding a patent long term.
Actually, this isn't a good example, as a new drug is such an established item that it's not really a new invention as such, and there's no reason they couldn't just be copyrighted (with less term).
Northwestern is obviously not the troll in this hypothetical, since it's the inventor, but the entity that buys Northwestern's patent may very well be a non-practicing entity. If a suit by Northwestern to enforce the patent would be legitimate, why not a suit by anyone else who bought the patent?
The point is that "using the property in your business" is not a requirement for legitimately enforcing property rights when it comes to any other kind of property. The right to use property productively is part of the "bundle of sticks" that defines a property right, but so is the right to sue for infringement of that property. Splitting up those individual rights and selling them and buying them individually, these are all just part of how property rights work.
The whole point of treating inventions as property is to allow inventors, like Northwestern, to monetize their inventions in the same ways they might monetize a piece of land. They can build something on it, sell it to someone else to build something on it, or even sell it to someone who does nothing with it but sues people who trespass on it. Usually people don't do the latter with land, but the nature of land makes for very different enforcement than the intellectual property.
Again, this is completely orthogonal to what you might think the scope of patent rights should be. Maybe we shouldn't treat them as property. Maybe they should go into the public domain immediately. That's all plausible. But we do treat them as property, and lawsuits are just one of the ways of monetizing property.
You can't avoid the issue that patents aren't property.
You are free to believe that patents encourage invention, but it's still government-granted monopoly, not property.
If patents are now more valuable to hold and sue, rather than use in product, than that's failed, and the solution is to make patents valid only when they're being used.
"If nature has made any one thing less susceptible than all others of exclusive property, it is the action of the thinking power called an idea, which an individual may exclusively possess as long as he keeps it to himself; but the moment it is divulged, it forces itself into the possession of every one, and the receiver cannot dispossess himself of it. Its peculiar character, too, is that no one possesses the less, because every other possesses the whole of it. He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me."
Property has existed since the very beginning of settled societies, throughout the world. Civil law is a later development that simply codified the concept.
There are very strong arguments for the existence of patents in areas that can be defined easily, for example drugs, materials, in fact I'm undecided there. The argument for patents is that they encourage both innovation, and resulting products. However a patent is still not property and thus there is no natural right to it.
Actually, this isn't a good example, as a new drug is such an established item that it's not really a new invention as such, and there's no reason they couldn't just be copyrighted (with less term).